A valuable class of photographic supports and elements comprises a paper base material having thereon a polyolefin coating containing a white pigment and an optical brightener. Such supports are particularly useful in the preparation of photographic elements such as color prints because they exhibit good brightness and excellent dimensional stability and are highly resistant to the action of aqueous acid and alkaline photographic processing solutions. The polyolefin coating provides a very smooth surface which is desirable when thin layers, such as silver halide emulsion layers, are to be coated thereover, U.S. Pat. No. 3,411,908 describes such a support which has achieved widespread commercial acceptance.
The purpose of the optical brightener is to make the white areas of the support appear even brighter. The optical brightener fluoresces upon irradiation with UV (ultraviolet) light, emitting visible light, usually bluish in hue, thus enhancing the brightness of the support. Optical brighteners for use in photographic print materials must absorb UV light, especially in the region from 360 to 420 nm, and reemit such light so as to enhance the brightness of the print, and have the desired brightening power. The optical brightener must also be stable to the temperatures, as high as 310.degree.-330.degree. C., used in extruding the polyolefin onto the paper base material.
Moreover, the optical brightener must be nonmigrating so that it remains in the polyolefin coating and does not exude as a surface film on the polyolefin. Such exudation not only can give rise to a nonuniform brightness of the reflection surface of the support, but also readily transfers to any other surface contacted with it. For example, brightener transferred nonuniformly to the back side of the adjacent layer of support results in visual imperfections. Brightener when migrated to the surface of the front side, can when wound in roll form, adversely affect subsequent coating and finishing operations and, in consequence, the quality and performance of the final element.
In general, prior art brighteners do not exhibit the combination of absorption/emission characteristics and brightening power, heat stability, and resistance to brightener exudation to the levels desired for photographic supports and elements. Thus, what has been desired is a photographic element comprising an optically brightened support, such support having improved resistance to brightener exudation and wherein the brightener exhibits excellent absorption/exmission characteristics, brightening power and heat stability.
Tomko et al, U.S. Pat. No. 4,794,071, provides a particularly efficacious photographic element containing a fluorescent bis(benzoxazolyl) stilbene optical brightener mixture that has a reduced tendency to migrate. The support provided by Tomko is especially useful for color prints, and is comprised of a paper base material having thereon a polyolefin coating containing a white pigment and a mixture of optical brighteners, such mixture comprising certain fluorescent bis(benzoxazolyl)-stilbenes. The support exhibits improved brightness at low brightener concentration and unexpected resistance to brightener exudation.
This invention provides, in one aspect, an improvement in the elements of Tomko et al. Thus, this invention provides an improved brightener which is complexed with a cyclodextrin. It also provides the use of such brightener inclusion compounds in photographic elements having a paper support. Complexed brighteners of this invention having a reduced tendency to migrate compared to the uncomplexed brighteners of Tomko et al.
In summary, it is desirable to have resin-coated paper used as a support for reflective photographic prints to have a bluish hue so that once the emulsion is placed on it a white D.sub.min results. In order to obtain a bluish tint, either pigments, dyes or optical brighteners may be added to the polyethylene. The method of adding the optical brighteners is preferred, since this does not result in a loss of brightness, which is the case for either pigments or dyes. However, it is known that many optical brighteners tend to migrate from the polyethylene, even at relatively low levels, due to limited solubility. This migration phenomenon is not desirable since it can cause non-uniform color of the support or may contaminate subsequent coating operations, or may cause the emulsion to not properly adhere to the support. Therefore, in the art, optical brighteners can only be used in limited amounts, and the rest of the bluish tint must be acquired using pigments and/or dyes.
Thus, it is desirable to improve D.sub.min in the support via use of an optical brightener that has a lessened tendency to migrate during prolonged storage or shipping, prior to sensitizing. This invention satisfies that need.